1. Field of the Invention
The present invention generally relates to a light source structure. More specifically, the present invention relates to a light source structure for a planar light-emitting device.
2. Background Information
FIG. 10 is an exploded cross sectional view of a conventional light source attachment structure. FIG. 11 is a cross sectional view of the conventional light source attachment structure. In FIGS. 10 and 11, an LED-mounted substrate 110 is faulted by sequentially laminating a base material 111, a copper plate 112 formed on the surface of the base material 111, and a resist film (e.g., insulation layer) 113 formed on substantially the entire surface of the base material 111 so as to cover the copper plate 112. In this LED-mounted substrate 110, a light-emitting diode (LED) 120 as a light source is soldered to the copper plate 112 by utilizing an opening 114 formed in the resist film 113. In other words, the LED 120 inserted into the opening 114 of the resist film 113 is soldered to the copper plate 112 by using solder 130. In FIG. 10, the reference numeral 121 refers to a thermal pad provided to the LED 120. The thermal pad 121 is placed in contact with the copper plate 112 by soldering of the LED 120 to the copper plate 112.
The LED-mounted substrate 110 on which the LED 120 is mounted is bonded to an inside surface 141 of a frame 140 which houses a light guide plate or other optical component not shown in the drawing, by using an adhesive tape 150.
In the light source attachment structure configured as described above, a heat transfer path for radiating the generated heat of the LED 120 is formed by the thermal pad 121, the solder 130, the base material 111, the adhesive tape 150, and the frame 140. Consequently, the thermal pad 121, the solder 130, the base material 111, and the adhesive tape 150 which form the heat transfer path are disposed on the inside of the frame 140. Furthermore, the base material 111 is made of a non-conductive material having low thermal conductivity. Thus, heat is easily trapped around the LED 120 in the internal space of the frame 140. Therefore, even when the frame 140 is metallic in order to increase the heat dissipation properties thereof, the trapping of heat around the LED 120 compromises the ability to increase the heat dissipation properties of the LED 120.
In the light source attachment structure described above, in the step of attaching the LED-mounted substrate 110 to the frame 140, the adhesive tape 150 must be used to bond the LED-mounted substrate 110 to the inside surface 141 of the frame 140 from the inside of the frame 140 which houses the optical component. Therefore, in this attachment step, bonding the LED-mounted substrate 110 to the inside surface 141 of the frame 140 while avoiding the installation location of the light guide plate or other optical component requires that the attachment operation be performed in a narrow space. Furthermore, damage is also prone to occur by interference of the LED 120 with other components.
In the light source attachment structure of the conventional example described with reference to FIG. 11, the base material 111 made of a non-conductive material having low thermal conductivity is interposed in the heat transfer path for radiating the generated heat of the LED 120. Thus, heat is easily trapped around the LED 120 in the internal space of the frame 140, as described above.
Another conventional light source attachment structure is also known. In the light source attachment structure, a metal base material, which is a material having good thermal conductivity, is used instead of the base material 111 of the LED-mounted substrate 110 shown in FIG. 11. However, when such metal base material is used, a thin electrical insulation layer must be interposed between the metal base material and the copper plate to which an LED is soldered. As a result, the interposition of the electrical insulation layer in the heat transfer path for radiating the generated heat of the LED compromises the ability to obtain adequate heat dissipation properties.
Further another conventional light source attachment structure has also been proposed. In the light source attachment structure, copper plates are formed on both the front and back surfaces of a base material that is the same as that used in the LED-mounted substrate 110 shown in FIG. 11. An LED is soldered to the copper plate of one side. Through-holes which penetrate through the base material are formed in numerous locations. With this light source attachment structure, the heat of the LED is readily transferred by air flowing through the through holes from the copper plate on the front side of the base material to the copper plate connectedly provided on the back side of the base material. Thus, heat dissipation effects are correspondingly improved. However, an adequate surface area for heat transfer to the copper plate on the back side of the base material is difficult to ensure merely by forming the through-holes, and the improvement to heat dissipation properties is limited.
Various proposals have been made for radiating generated heat of a light source LED (see Japanese Laid-Open Patent Application Publications Nos. 2009-169204, 2008-166304 and 2006-308738, for example).
For example, a light source attachment structure is employed in a liquid crystal display device (see Japanese Laid-Open Patent Application Publication No. 2009-169204, for example). In this light source attachment structure, an LED-mounted substrate is formed by mounting an in-line arranged LED on a flexible printed substrate (FPC) 101. The LED-mounted substrate is attached to a heat conducting plate made of aluminum. The heat conducting plate is attached in a state of contact with an inside surface of a casing. In the LED-mounted substrate, the LED is mounted in a state of penetrating through the FPC. The LED contacts with the heat conducting plate.
With this light source attachment structure, the generated heat of the LED is efficiently transferred to the casing through the heat conducting plate. Thus, the heat dissipation properties of the LED can also be enhanced. However, the LED-mounted substrate is disposed inside the casing. Thus, the step of attaching the LED-mounted substrate to the inside surface of the casing requires that the attachment operation be performed in a narrow space. As a result, damage is also prone to occur by interference of the LED with other components. The LED-mounted substrate is also disposed inside the casing. Thus, heat around the LED is easily trapped in the internal space of the casing. Therefore, in the light source attachment structure, a transparent plate having high thermal conductivity, attached to the casing, is disposed in the space between the LED and a light guide plate. In this structure, the heat trapped around the LED in the internal space of the casing is allowed to escape through the transparent plate. However, providing the transparent plate causes problems in that the intensity of the light introduced to the light guide plate from the LED is reduced by passing through the transparent plate. Thus, the efficiency with which the light of the LED is utilized is reduced.
Another conventional structure includes a thermal via or other through-hole for heat dissipation in a wiring substrate on which an LED is mounted (see Japanese Laid-Open Patent Application Publication No. 2008-166304, for example). With further another conventional structure, an LED-mounted substrate is bonded to a heat sink through the use of a thermally conductive adhesive. However, the above-mentioned techniques require attaching an LED to the inside surface of a frame. Consequently, it has been discovered that with the techniques, the problem of reduced operating efficiency of attachment, and the problem of heat being easily trapped in the internal space of the frame are impossible to avoid.